Won't go into your Q about where to carry. As for something more sufficient than just stuffing your firearm under your seat these are available just about everywhere for around $30:

Believe it or not, if you can train yourself to use one of these, they're actually pretty darn secure for general use. If someone just happened to come across it during a quick burglary, or while parked in a parking lot, if they didn't happen to have a substantial wire cutter I doubt they'd be able to remove it in any reasonable amount of time. And if they did somehow, they literally have to destroy the box to get it open, with a good chance the firearm would be damaged as well.